Question

the space between the iner walls of a thermos bottle (sometimes called a dewar flask) is ecavuated to remove the air that would otherwise be there. why is this evacuation done?

Answer 1

The air is evacuated from a thermos to create a vacuum, reducing conduction and convection, while silvered walls reflect heat, and other design features like the long neck and stopper further insulate the contents.

Step-by-step explanation:

The evacuation of air from the space between the inner walls of a thermos bottle (a Dewar flask) is done to minimize heat transfer. The vacuum created by this evacuation greatly reduces the conduction and convection of heat, as these processes require matter to transfer energy. Moreover, the silvering of the walls reflects heat radiation, which further prevents heat loss or gain. The thin-walled long glass neck minimizes the area where heat can be transferred by conduction. Rubber supports diminish the transfer of heat through conduction as well. The air layer above the liquid acts as an additional insulation, and the stopper minimizes heat transfer via convection and conduction by sealing the opening of the thermos.